According to most existing telecommunications standards, the transmission of speech information, in particular when transmitted over a wireless interface, takes the form of compressed speech parameters. Upon receipt of compressed speech parameters at a base station in communication with a mobile unit, the speech parameters are processed by a codec (coder/decoder), which converts (expands) the speech parameters into speech samples in order to provide compatibility with the public switched telephone network (PSTN). The speech samples are then transmitted over the PSTN.
If the other party is connected directly to the PSTN, such as via a wireline connection, the speech samples having traveled through the network will simply be converted into audio form by a digital telephone unit at the other party site. Of course, the other party may also be a second mobile unit, in which case the speech samples will terminate at a second base station, where a second codec re-converts the speech samples back into compressed speech parameters for transmission to the second mobile unit via a wireless interface. The usage of a source decoder to expand speech parameters into a stream of speech samples, in combination with the use of a destination encoder for re-compression of these samples into a second set of compressed speech parameters, is referred to as operation of codecs in tandem, or “tandem operation”.
Those skilled in the art will appreciate that the tandem operation described above introduces a degradation in service quality, as errors may be introduced by the decompression and re-compression operations performed by the source and destination codecs, respectively. When the codec are compatible, such error should in principle be avoidable, as neither codec operation is required by virtue of the second base station requiring the compressed speech parameters rather than the expanded speech samples. Furthermore, the transmission of speech samples in an uncompressed format has greater bandwidth requirements than that of the compressed format.
A method for reducing the occurrence of tandem codecs, called Tandem-Free Operation (TFO), uses an in-band handshaking protocol to detect the presence of tandem codecs, and then bypasses the codecs to transmit the compressed speech parameters. This arrangement bypasses the requirement for decompression at the source codec and (re-)compression at the destination codec, which obviates the occurrence of errors at these two stages. As a result, a high quality of service can be achieved for a given end-to-end call between two mobile units.
A more recent approach, called Transcoder-Free Operation (TrFO), uses out-of-band signaling to detect call scenarios involving tandem codecs at call set-up time. Thereupon action is taken to put in place a direct end-to-end link to provide for a direct exchange of the compressed speech parameters without the involvement of network transcoders.
For information on the TFO and TrFO techniques, the reader is invited to refer to the following documents that are hereby incorporated by reference:                3rd generation partnership project, Technical specification group core network, Out of band transcoder control—Stage 2 (3GPP TS 23.153 V4.4.0 (2001–12));        3rd generation partnership project, Technical specification group core network, Bearer-independent circuit-switched core network, Stage 2 (3GPP TS 23.205 V4.4.0 (2002–03));        3rd generation partnership project, Technical specification group (TSG) RAN3, Transcoder free operation (3GPP TR 25.953 V4.0.0 (2001–03));        3rd generation partnership project, Technical specification group services and system aspects, Inband tandem free operation (TFO) of speech codecs, service description—Stage 3 (3GPP TS 28.062 V5.0.0 (2002–03)).        
A deficiency of the above noted methods is that they require the source codec and the destination codec for each end of the communication path to be compatible. For example, with reference to FIG. 1, mobile module 800 and mobile unit 818 each make use of a respective codec type and codec types are incompatible with one another. Mobile module 800 communicates with gateway 804 and mobile module 818 communicates with gateway 808. If gateway 804 wants to establish a tandem free connection with gateway 808, gateway 804 starts a handshaking protocol and sends control messages to the gateway 808. This control information can be in the form of in-band signalling or in the form out-of-band signalling. Although a negotiation can take place between the gateways 804 and 808, a direct tandem free connection cannot be established because the mobiles 800 and 818 use incompatible encoders and decoders. Consequently, when the handshaking protocol determines that the codecs are incompatible the speech parameters are processed by the source codec (coder/decoder) in the gateway which converts (expands) the speech parameters into speech samples and are transmitted to the destination codec in the other gateway. Consequently, uncompressed speech samples are transmitted over communication link 806 which requires a greater amount of bandwidth than the transmission of compressed speech parameters. This increase in bandwidth requirement is undesirable.
Thus there exists a need in the industry for providing an improved communication apparatus that alleviates at least in part problems associated to the prior art.